1. Technical Field
The present application relates to an optical microphone which is configured to receive an acoustic wave propagating through a gas, such as air, or an acoustic wave propagating through a solid, and convert the received acoustic wave to an electric signal using a light wave.
2. Description of the Related Art
A conventionally-known device for detecting an acoustic wave is a microphone. Many microphones, typified by dynamic microphones and condenser microphones, use a diaphragm. In these microphones, an input acoustic wave vibrates the diaphragm, and the vibration is extracted as an electric signal by means of the piezoelectric effect or a variation in electric capacity. An optical microphone which is configured to detect the vibration of the diaphragm using a light wave, such as a laser beam, is also known.
On the other hand, Japanese Laid-Open Patent Publication No. 2009-085868 (hereinafter, referred to as “Patent Document 1”) discloses an optical microphone which is configured to detect an acoustic wave by means of a light wave, without using a diaphragm. As shown in FIG. 35, the optical microphone disclosed in Patent Document 1 includes an acousto-optic medium section 203 and a laser Doppler vibrometer 204. The acousto-optic medium section 203 is supported inside a recessed portion of a base 210, and the opening of the recessed portion is covered with a transparent plate 211. The base 210 has an opening portion 201. The opening portion is provided with a space that functions as an acoustic waveguide 202 which is formed by a lateral surface 203a of the acousto-optic medium section 203 and an inside surface of the recessed portion of the base 210.
An acoustic wave 205 propagating in the air is taken into the base 210 from the opening portion 201 so as to travel through the acoustic waveguide 202. The acoustic wave 205 is taken into the inside of the acousto-optic medium section 203 from the lateral surface 203a so as to propagate through the acousto-optic medium section 203.
In the acousto-optic medium section 203, propagation of the acoustic wave 205 causes a variation in refractive index. This refractive index variation is extracted by the laser Doppler vibrometer 204 as optical modulation, whereby the acoustic wave 205 is detected. Using a silica nanoporous element (dry silica gel) as the acousto-optic medium section 203 enables the acoustic wave 205 propagating in the acoustic waveguide 202 to be taken into the inside of the acousto-optic medium section 203 with high efficiency.